Lead including conductors configured for reduced MRI-induced currents

ABSTRACT

An implantable medical device lead includes an inner conductor coil comprising one or more generally cylindrically wound filars. The inner conductor coil is configured to have a first inductance value greater than or equal to 0.2 μH/inch when the inner conductor coil is subjected to a range of radio frequencies. The implantable medical device lead also includes a multi-filar outer coil comprising two or more generally cylindrically wound filars. The multi-filar outer coil is configured to have a second inductance value greater than or equal to 0.1 μH/inch when the multi-filar outer coil is subjected to the range of radio frequencies.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/030,467, filed Feb. 18, 2012, which claims the benefit of ProvisionalApplication No. 61/306,377, filed Feb. 19, 2010, which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

Various embodiments of the present invention generally relate toimplantable medical devices. More specifically, embodiments of thepresent invention relate to conductor configurations for magneticresonance imaging (MRI) compatibility.

BACKGROUND

When functioning properly, the human heart maintains its own intrinsicrhythm and is capable of pumping adequate blood throughout the body'scirculatory system. However, some individuals have irregular cardiacrhythms, referred to as cardiac arrhythmias, which can result indiminished blood circulation and cardiac output. One manner of treatingcardiac arrhythmias includes the use of a pulse generator, such as apacemaker, an implantable cardioverter defibrillator (ICD), or a cardiacresynchronization (CRT) device. Such devices are typically coupled to anumber of conductive leads having one or more electrodes that can beused to deliver pacing therapy and/or electrical shocks to the heart. Inatrioventricular (AV) pacing, for example, the leads are usuallypositioned in a ventricle and atrium of the heart, and are attached vialead terminal pins to a pacemaker or defibrillator which is implantedpectorally or in the abdomen.

Magnetic resonance imaging (MRI) is a non-invasive imaging procedurethat utilizes nuclear magnetic resonance techniques to render imageswithin a patient's body. Typically, MRI systems employ the use of amagnetic coil having a magnetic field strength of between about 0.2 to 3Teslas. During the procedure, the body tissue is briefly exposed to RFpulses of electromagnetic energy in a plane perpendicular to themagnetic field. The resultant electromagnetic energy from these pulsescan be used to image the body tissue by measuring the relaxationproperties of the excited atomic nuclei in the tissue. In some cases,imaging a patient's chest area may be clinically advantageous. In achest MRI procedure, implanted pulse generators and leads may also beexposed to the applied electromagnetic fields.

SUMMARY

Various embodiments of the present disclosure generally relate toimplantable lead conductor configurations for magnetic resonance imaging(MRI) compatibility.

In Example 1, an implantable medical device lead includes an innerconductor coil comprising one or more generally cylindrically woundfilars. The inner conductor coil is configured to have a firstinductance value greater than or equal to 0.2 μH/inch when the innerconductor coil is subjected to a range of radio frequencies. Theimplantable medical device lead also includes a multi-filar outer coilcomprising two or more generally cylindrically wound filars. Themulti-filar outer coil is configured to have a second inductance valuegreater than or equal to 0.1 μH/inch when the multi-filar outer coil issubjected to the range of radio frequencies.

In Example 2, the implantable medical device according to Example 1,wherein a layer of insulation is disposed about at least a portion ofthe inner conductor coil.

In Example 3, the implantable medical device according to either Example1 or Example 2, wherein the inner conductor coil has a unifilarconstruction.

In Example 4, the implantable medical device according to any ofExamples 1-3, wherein the inner conductor coil has an average pitch ofapproximately 0.005 inches.

In Example 5, the implantable medical device according to any ofExamples 1-4, wherein the inner conductor coil has a unifilarconstruction and a mean coil diameter of 0.023 inches.

In Example 6, the implantable medical device according to any ofExamples 1-5, wherein the inner conductor coil has an inductance greaterthan approximately 0.5 μH/inch.

In Example 7, the implantable medical device according to any ofExamples 1-6, wherein the first inductance value (L) is set by a numberof cylindrically wound filars (N), a pitch (b) of the inner conductorcoil, and a mean coil diameter (a) by the equation

$L \approx \frac{\mu_{0}\pi\; a^{2}}{4b^{2}N^{2}}$where μ0 is the permeability of the free space.

In Example 8, the implantable medical device according to any ofExamples 1-7, wherein the inner conductor coil has a DC resistance lessthan 200 ohms.

In Example 9, the implantable medical device according to any ofExamples 1-8, wherein the multi-filar outer coil is a ribbon-typeconductor coil.

In Example 10, a medical lead includes a flexible body having a proximalregion with a proximal end, and a distal region. A connector coupled tothe proximal end of the body is configured for electrically andmechanically connecting the lead to an implantable pulse generator. Aninner conductor coil is configured to convey electrical signals betweena distal section and a proximal section of the lead, and the low voltageinner conductor coil includes one or more generally cylindrically woundfilars. The inner conductor coil is configured to have a firstinductance greater than or equal to 0.2 μH/inch when subjected to radiofrequencies between 40 megahertz (MHz) and 300 MHz. A multi-filar outerconductor coil including two or more generally cylindrically woundfilars radially surround at least a portion of the low voltage innerconductor coil. The multi-filar outer conductor coil is configured tohave a second inductance greater than or equal to 0.1 μH/inch whensubjected to the radio frequencies between 40 megahertz (MHz) and 300MHz.

In Example 11, the medical lead according to Example 10, and furthercomprising a tri-filar shocking coil with a proximal end, wherein theproximal end of the tri-filar shocking coil is connected via a couplerto a distal end of the multi-filar outer coil.

In Example 12, the medical lead according to Example 11, wherein themulti-filar outer coil has an outer diameter larger than an outerdiameter of the tri-filar shocking coil.

In Example 13, the medical lead according to any of Examples 10-12,wherein the multi-filar outer conductor comprises a quad-filar coilhaving a helix-like shape.

In Example 14, the medical lead according to any of Examples 10-13,wherein the lead further includes one or more layers of insulatingmaterial surrounding one or both of the inner conductor coil and themulti-filar outer conductor coil.

In Example 15, the medical lead according to any of Examples 10-14,wherein the inner conductor coil and the multi-filar outer conductorcoil have different pitches.

In Example 16, the medical lead according to any of Examples 10-15,wherein the inner conductor coil and the multi-filar outer conductorcoil each have a pitch no greater than about 0.005 inch (0.127 mm).

In Example 17, an implantable medical device lead includes an innerconductor coil comprising one or more wound filars and configured tohave a first inductance value greater than or equal to 0.2 μH/inch whensubjected to radio frequencies between 40 megahertz (MHz) and 300 MHz.The implantable medical device lead also includes a multi-filar outerconductor coil comprising two or more generally cylindrically woundfilars radially surrounding at least a portion of the inner conductorcoil and configured to have a second inductance value greater than 0.1μH/inch when subjected to the radio frequencies between 40 megahertz(MHz) and 300 MHz. The implantable medical device lead further includesa tri-filar shocking coil with a proximal end, wherein the proximal endis connected via a coupler to a distal end of the multi-filar outerconductor coil.

In Example 18, the implantable medical device according to Example 17,wherein the inner conductor coil, the multi-filar outer conductor coil,and the tri-filar shocking coil have different pitches.

In Example 19, the implantable medical device according to eitherExample 17 or Example 18, wherein the lead further includes one or morelayers of insulating material surrounding one or more of the low voltageinner conductor coil, the multi-filar high voltage outer conductor coil,and the tri-filar shocking coil.

In Example 20, the implantable medical device according to any ofExamples 17-19, wherein a pitch of the multi-filar outer conductor coilis about 0.010 inches, a mean coil diameter of the multi-filar outerconductor coil is about 0.090 inches and results in a coil inductancevalue of about 0.13 μH/inch, and a pitch of the inner conductor coil isabout 0.005 inches, the inner conductor coil is formed from onecylindrically wound filar, and a mean coil diameter of the innerconductor coil is about 0.023 inches resulting in a coil inductance perunit length value of about 0.5 μH/inch.

While multiple embodiments are disclosed, still other embodiments of thepresent invention will become apparent to those skilled in the art fromthe following detailed description, which shows and describesillustrative embodiments of the invention. Accordingly, the drawings anddetailed description are to be regarded as illustrative in nature andnot restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a medical system including an MRIscanner, and an implantable cardiac rhythm management system implantedwithin a torso of a human patient according to various embodiments ofthe present invention;

FIG. 2A is a schematic view of an illustrative pulse generator and leadimplanted within the body of a patient which may be used in accordancewith some embodiments of the present invention;

FIG. 2B is a schematic view showing a simplified equivalence circuit forthe lead of FIG. 2A

FIG. 3 illustrates an exemplary lead that may be used in accordance withone or more embodiments of the present invention;

FIG. 4 illustrates a cross-sectional view of a high voltage shockingcoil and a low voltage coil in accordance with various embodiments ofthe present invention;

FIGS. 5A-5C show various portions of the inner conductor coil, the highvoltage conductor coil, and the shocking coil according to someembodiments of the present invention;

FIG. 6 illustrates an example of a resulting temperature increase when astandard lead design and an exemplary lead designed according to variousembodiments of the present invention when the standard lead design andthe exemplary lead design are subjected to MRI related frequencies; and

FIG. 7 is a transverse cross-sectional view of a lead with a multi-lumenconstruction that may be used in some embodiments of the presentinvention.

The drawings have not necessarily been drawn to scale. For example, thedimensions of some of the elements in the figures may be expanded orreduced to help improve the understanding of the embodiments of thepresent invention. While the invention is amenable to variousmodifications and alternative forms, specific embodiments have beenshown by way of example in the drawings and are described in detailbelow. The intention, however, is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION

An implantable cardioverter defibrillator (ICD) is typically implantedin the pectoral region of a patient. In some cases, one or twoelectrodes may extend from the ICD into an atrium and/or ventricle ofthe patient's heart. In the case of epicardial leads, the electrodes areattached to an external surface of the patient's heart. The ICD systemcan provide pacing capability to the patient's heart and/or a highvoltage shocking therapy to convert patient's heart from fibrillation tonormal heart function.

As explained in further detail below, various embodiments of the presentinvention relate to new lead designs advantageously adapted foroperation in a magnetic resonance imaging (MRI) environment. In someembodiments, the leads include combinations of unique shocking coilsand/or coil conductors configured to provide suitable electricalperformance for tachycardia therapy and also to minimize the lead'sreaction to applied electromagnetic energy during MRI procedures.

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of embodiments of the present invention. It will beapparent, however, to one skilled in the art that embodiments of thepresent invention may be practiced without some of these specificdetails.

While, for convenience, some embodiments are described with reference toICDs in the presence of MRI scanners. Embodiments of the presentinvention may be applicable to various other physiological measurements,treatments, implantable medical devices, and other non-invasiveexamination techniques in which conductive leads are exposed to timevarying magnetic fields. As such, the applications discussed herein arenot intended to be limiting, but instead exemplary. Other systems,devices, and networks to which embodiments are applicable include, butare not limited to, other types of sensory systems, medical devices,medical treatments, and computer devices and systems. In addition,various embodiments are applicable to all levels of sensory devices froma single IMD with a sensor to large networks of sensory devices.

FIG. 1 is a schematic illustration of a medical system 100 including aMRI scanner 110, an implantable cardiac rhythm management (CRM) system115 implanted within a torso of a human patient 120, and one or moreexternal device(s) 130 according to various embodiments. The externaldevice(s) 130 are capable of communicating with the CRM system 115implanted within the patient 120. In the embodiment shown in FIG. 1, theCRM system 115 includes a pulse generator (PG) 140 and a lead 150.During normal device operation, the PG 140 is configured to deliverelectrical therapeutic stimulus to the patient's heart 160 for providingtachycardia ventricular fibrillation, anti-bradycardia pacing,anti-tachycardia pacing, and/or other types of therapy.

Thus, in the illustrated embodiment, the PG 140 can be a device such asan ICD, cardiac resynchronization therapy device with defibrillationcapabilities (a CRT-D device), or a comparable device. The PG 140 can beimplanted pectorally within the body, typically at a location such as inthe patient's chest. In some embodiments, PG 140 can be implanted in ornear the abdomen.

The external devices 130 may be a local or remote terminal or otherdevice (e.g., a computing device and/or programming device), operable tocommunicate with the PG 140 from a location outside of the patient'sbody. According to various embodiments, external device 130 can be anydevice external to the patient's body that is telemetry enabled andcapable of communicating with the PG 140. Examples of external devicescan include, but are not limited to, programmers (PRM), in-homemonitoring devices, personal computers with telemetry devices, MRIscanner with a telemetry device, manufacturing test equipment, or wands.In some embodiments, the PG 140 communicates with the remote terminal130 via a wireless communication interface. Examples of wirelesscommunication interfaces can include, but are not limited to, radiofrequency (RF), inductive, and acoustic telemetry interfaces.

FIG. 2A is a more detailed schematic view of the CRM system 115including the illustrative PG 140 equipped with the lead 150 implantedwithin the body of a patient. In the embodiments depicted, CRM system115 includes a PG implanted near the patient's heart 160 and lead 150having a distal portion implanted with the patient's heart 160. As canbe seen in FIG. 2A, the heart 160 includes a right atrium 210, a rightventricle 220, a left atrium 230, and a left ventricle 240.

The lead 150 has a flexible body 200 including a proximal region 205 anda distal region 250. As shown, the lead 150 is coupled to the PG 140,and the distal region 250 of the lead body 200 is at least partiallyimplanted at a desired location within the right ventricle 220. Asfurther shown, the lead 150 includes at least one electrode 255 alongthe distal region 250, such that when implanted as shown in FIG. 2A, itis positioned within the right ventricle 220. As explained andillustrated in further detail below, the lead 150 includes one or moreelectrical conductor coils within the lead body 250 (not visible in FIG.2A) electrically coupling the electrode 255 to circuitry and otherelectrical components within the PG 140 for transmitting intrinsiccardiac signals from the heart 160 to the PG 140 and also fortransmitting electrical shocks or low-voltage pacing stimuli to theheart 160 via the electrode 255.

Although the illustrative embodiment depicts only a single lead 150inserted into the patient's heart 160, in other embodiments multipleleads can be utilized so as to electrically stimulate other areas of theheart 160. In some embodiments, for example, the distal portion of asecond lead (not shown) may be implanted in the right atrium 210. Inaddition, or in lieu, another lead may be implanted at the left side ofthe heart 160 (e.g., in the coronary veins, the left ventricle, etc.) tostimulate the left side of the heart 160. Other types of leads such asepicardial leads may also be utilized in addition to, or in lieu of, thelead 150 depicted in FIGS. 1-2.

During operation, the lead 150 conveys electrical signals between theheart 160 and the PG 140. For example, in those embodiments where the PG140 has pacing capabilities, the lead 150 can be utilized to deliverelectrical therapeutic stimulus for pacing the heart 160. In thoseembodiments where the PG 140 is an ICD, the lead 150 can be utilized todeliver high voltage electric shocks to the heart 160 via the electrode255 in response to an event such as a ventricular fibrillation. In someembodiments, the PG 140 includes both pacing and defibrillationcapabilities.

FIG. 2B is a schematic view showing a simplified equivalence circuit 260for the lead 150 of FIG. 2A, representing the RF energy picked up on thelead 150 from RF electromagnetic energy produced by an MRI scanner. Asshown in FIG. 2B, voltage (Vi) 265 in the circuit 260 represents anequivalent source of energy picked up by the lead 150 from the MRIscanner. During magnetic resonance imaging, the length of the lead 150functions similar to an antenna, receiving the RF energy that istransmitted into the body from the MRI scanner. Voltage (Vi) 265 in FIG.2B may represent, for example, the resultant voltage received by thelead 150 from the RF energy. The RF energy picked up by the lead 150 mayresult, for example, from the rotating RF magnetic field produced by anMRI scanner, which generates an electric field in the planeperpendicular to the rotating magnetic field vector in conductivetissues. The tangential components of these electric fields along thelength of the lead 150 couple to the lead 150. The voltage (Vi) 265 isthus equal to the integration of the tangential electric field (i.e.,the line integral of the electric field) along the length of the lead150.

The Zl parameter 270 in the circuit 260 represents the equivalentimpedance exhibited by the lead 150 at the RF frequency of the MRIscanner. The impedance value Zl 270 may represent, for example, theinductance or the equivalent impedance resulting from the parallelinductance and the coil turn by turn capacitance exhibited by the lead150 at an RF frequency of 64 MHz for a 1.5 Tesla MRI scanner, or at anRF frequency of 128 MHz for a 3 Tesla MRI scanner. The impedance Zl ofthe lead 150 is a complex quantity having a real part (i.e., resistance)and an imaginary part (i.e., reactance).

Zb 275 in the circuit 260 may represent the impedance of the body tissueat the point of lead contact. Zc 280, in turn, may represent thecapacitive coupling of the lead 150 to surrounding body tissue along thelength of the lead 150, which may provide a path for the high frequencycurrent (energy) to leak into the surrounding tissue at the RF frequencyof the MRI scanner. Minimizing the absorbed energy (represented bysource Vi 265) reduces the energy that is transferred to the body tissueat the point of lead contact with the body tissue.

As can be further seen in FIG. 2B, the lead 150 has some amount ofleakage into the surrounding tissue at the RF frequency of the MRIscanner. As further indicated by 275, there is also an impedance at thepoint of contact of the lead electrode(s) 255 to the surrounding bodytissue within the heart 160. The resulting voltage Vb delivered to thebody tissue may be related by the following formula:Vb=ViZbe/(Zbe−Zl),where Zbe=Zb in parallel with Zc.

The temperature at the tip of the lead 150 where contact is typicallymade to the surrounding tissue is related in part to the powerdissipated at 275 (i.e., at “Zb”), which, in turn, is related to thesquare of Vb. To minimize temperature rises resulting from the powerdissipated at 275, it is thus desirable to minimize Vi (265) and Zc(280) while also maximizing the impedance Zl (270) of the lead 150. Insome embodiments, the impedance Zl (270) of the lead 150 can beincreased at the RF frequency of the MRI scanner, which aids in reducingthe energy dissipated into the surrounding body tissue at the point ofcontact 275.

In the various embodiments described in further detail below, theimpedance of the lead 150 can be increased by adding inductance to thelead 150 and/or by a suitable construction technique. For example, invarious embodiments, the inductance of the lead 150 is increased byincreasing the mean diameter of the conductor coil(s) and/or bydecreasing the pitch of the conductor coil(s) used to supply electricalenergy to the electrode(s) 255. Decreasing the coil pitch may result inincreasing capacitance between successive turns of the coil (i.e., coilturn by turn capacitance). The parallel combination of inductance (fromthe helical shape of the coil) and the turn by turn capacitanceconstitutes a resonance circuit. For a helically coiled leadconstruction, if the resonance frequency of the lead is above the RFfrequency of the MRI, then the helical coil acts as an inductor. For aninductor, increasing the cross section of the coil area and/or reducingthe coil pitch increases the inductance and, as a result, increases theimpedance of the lead 150.

FIG. 3 schematically illustrates in further detail the exemplary lead150 that may be used in accordance with one or more embodiments of thepresent invention. In FIG. 3, a portion of the lead body 200 is shownpartially cut-away to better illustrate the internal features of thelead 150. As shown in FIG. 3, the lead body 200 includes a proximal end302, and the lead 150 further includes a connector assembly 310 coupledto the proximal end 302 of the lead body, a high voltage shockingconductor coil 320, a shocking coil 330, an inner conductor coil 340, acoupler 350, and pace/sense electrode 360. Depending on the functionalrequirements of the IMD 140 (see FIG. 1), and the therapeutic needs ofthe patient, the distal region may include additional shocking coils(not shown) and/or pace/sense electrodes. For example, in someembodiments, a pair of coil electrodes can be used to function asshocking electrodes for providing a defibrillation shock to the heart160.

In the illustrated embodiment, the connector assembly 310 includes aconnector body 365 and a terminal pin 370. The connector assembly 310 iscoupled to the lead body and can be configured to mechanically andelectrically couple the lead to a header on PG 140 (see FIG. 1). Invarious embodiments, the terminal pin 370 extends proximally from theconnector body 365 and in some embodiments is coupled to the innerconductor coil 340 that extends longitudinally through the lead body 200to the pace/sense electrode 360. In the illustrated embodiment, thepace/sense electrode 360 is a tip electrode located at the distal-mostextremity of the lead 150, and is fixed relative to the lead body 200such that the lead 150 is considered a passive-fixation lead. In otherembodiments, the lead 150 may include additional pace/sense electrodeslocated more proximally along the lead 150. In some embodiments, theterminal pin 370 can include an aperture extending therethroughcommunicating with a lumen defined by the inner conductor coil 340 inorder to accommodate a guide wire or an insertion stylet.

In some embodiments, the pace/sense electrode 360 may be in the form ofan electrically active fixation helix at the distal end of the lead 150.In various such embodiments, the pace/sense electrode 360 can be anextendable/retractable helix supported by a mechanism to facilitatelongitudinal translation of the helix relative to the lead body as thehelix is rotated. In those embodiments, the terminal pin 370 may berotatable relative to the connector body 365 and the lead body 200 suchthat rotation of the terminal pin 370 relative to the lead body 200causes the inner conductor coil 340, and in turn, the helical pace/senseelectrode 360 to rotate and translate longitudinally relative to thelead body 200. Various mechanisms and techniques for providingextendable/retractable fixation helix assemblies (both electricallyactive and passive) are known to those of ordinary skill in the art, andneed not be described in greater detail here.

The pace/sense electrode 360 (whether a solid tip electrode such asshown in FIG. 3 or an active-fixation helix as described above) can bemade of any suitable electrically conductive material such as Elgiloy,MP35N, tungsten, tantalum, iridium, platinum, titanium, palladium,stainless steel, as well as alloys of any of these materials.

The inner conductor coil 340 can be a relatively low voltage conductorto carry the pacing and sensing signal to and from the heart 160. Lowvoltage inner conductor coil 340 can be formed, according to variousembodiments, from one or more generally cylindrically wound filars. Asexplained in further detail below, in some embodiments, the low voltageinner conductor coil 340 is configured to have an inductance valuegreater than or equal to 0.2 μH/inch to reduce the RF current induced inthe inner conductor coil 340 due to an external MRI field, and also toprevent undesired high-rate stimulation of the heart. In someembodiments, the inductance value is around 0.5 μH/inch. Additionally,the inner conductor coil 340, in some embodiments, is configured to havea DC resistance less than 200 ohms.

In some embodiments, the high voltage conductor coil 320 can provide ahigh voltage path that can deliver up to 1000 volts and 40 J of energyto the patient's heart 160, as may be necessary for applying ananti-tachycardia shock. The high voltage conductor coil 320 isconfigured, in the various embodiments, to have a high inductance toreduce the current that induced by the RF pulses generated by an MRIdevice or other system. In some embodiments, the inductance is greaterthan or equal to 0.1 μH/inch. The outer diameter can be increased tomake up for a loss of inductance in some embodiments. According to oneor more embodiments, the high voltage conductor coil 320 may have amulti-filar construction to decrease the direct current (DC) resistance.In some embodiments, the DC resistance will be below approximately tenohms (e.g., six or seven ohms in some embodiments), so that the maximumenergy can be delivered to the heart.

In some embodiments, the high voltage coil 320 can be divided into twopaths; one path can be connected to a shocking coil that is proximal tothe ICD. Another path can be connected to a shocking coil that is distalto the ICD. The distal shocking coil, in conjunction with the highvoltage coil 320, can serve as the returning path of the pacing pulse inbipolar pacing. Alternatively, a second high voltage path may beprovided via a high voltage coil (not shown) separate from the highvoltage coil 320.

In some embodiments, the high voltage conductor coil 320 is mechanicallyand electrically coupled to the shocking coil 330 via coupler 350. Thispath can also serve as the returning path of the pacing pulse in bipolarpacing. The shocking coil 330 can also deliver proper therapy to thepatient's heart. Examples of therapy include, but are not limited to,tachycardia ventricular fibrillation, anti-bradycardia pacing,anti-tachycardia pacing, and/or other types of therapy.

In some embodiments, the shocking coil 330 may have a coating that isconfigured to control (i.e. promote or discourage) tissue in-growth. Invarious embodiments, the lead may include only a single coil electrodesuch as shocking coil 330. In other embodiments, the lead 150 mayinclude one or more ring electrodes (not shown) along the lead body inlieu of or in addition to the shocking coil electrodes 330. Whenpresent, the ring electrodes may operate as relatively low voltagepace/sense electrodes. As will be appreciated by those skilled in theart, a wide range of electrode combinations may be incorporated into thelead 150 within the scope of the various embodiments of the presentinvention.

FIG. 4 illustrates a cross-sectional view 400 of the lead 150 takenalong the line 4A in FIG. 3. As shown in FIG. 4, in the illustratedembodiment, the high voltage conductor coil 320 and the low voltageinner conductor coil 340 are coaxially disposed within the lead body200. As further shown, in the illustrated embodiment, the lead 150includes an insulation layer 410 between the high voltage conductor coil320 and the low voltage inner conductor coil 340 so as to electricallyisolate these coils from one another. In various embodiments, theindividual filars of the high voltage conductor coil 320 and/or the lowvoltage inner conductor coil 340 may be individually insulated inaddition to or in lieu of using the insulation layer 410. Accordingly,in the embodiment shown in FIG. 4, the filars of the high voltageconductor coil 320 and the low voltage inner conductor coil 340 eachhave a thin layer of insulation. In other embodiments, the filars of thehigh voltage conductor coil 320 and/or the low voltage inner conductorcoil 340 are not individually insulated, and are spaced apart to avoidcontact with adjacent filars.

Examples of the types of insulation material that can be used in variousembodiments of the present invention include, but are not limited to,silicone, polytetrafluoroethylene, expanded polytetrafluoroethylene,ethylene tetrafluoroethylene, and co-polymers of the foregoing. In someembodiments, the insulating layer of the individual filars can preventthe turns of the coil from coming in contact with each other when anuncoiled lead is placed in a helix configuration as shown in FIGS.5A-5C. In addition, some embodiments include a sufficient insulationlayer between the low voltage coil and high voltage coil to preventelectrical coupling.

As explained above, according to various embodiments of the presentinvention, the high voltage conductor coil 320 and/or the low voltageinner conductor coil 340 are selectively configured to have a highimpedance to minimize the effects of applied MRI radiation withoutunduly impacting electrical performance under normal operatingconditions (e.g., for providing anti-tachycardia therapy). As explainedin further detail below, in various embodiments, the filar thickness,pitch, and/or mean coil diameter for the high voltage conductor coil 320and/or the low voltage inner conductor coil 340 are selectively chosento provide the desired balance of electrical operating performance andMRI-compatibility.

FIGS. 5A-5C show various configurations of the inner conductor coil 340,the high voltage conductor coil 320, and the shocking coil 330,respectively, according to some embodiments of the present invention.According to various embodiments, the pitch of a helix is the width ofone complete helix turn, measured along the helix axis. Distances 510a-510 c in FIGS. 5A-5C illustrate the pitch of the coils shown,reference numerals 520 a-520 c represent the filar thickness for therespective coils, and reference numeral 530 a-530 c represents the meancoil diameter. According to one or more embodiments, the pitch may be aconstant pitch along the length of the lead (see, e.g., FIG. 5C) orfollow a repeating pattern along the length of the lead (see, e.g.,FIGS. 5A and 5B). In some embodiments, a ribbon type conductor may beused for the high voltage conductor coil 320 and the shocking coil 330.In some embodiments, the pitch directions for coils 320, 330, 340 arethe same.

FIG. 5A illustrates a portion of the high voltage conductor coil 320 inaccordance with some embodiments of the present invention. In theillustrated embodiment, the high voltage conductor coil 320 is aquad-filar coil. However, in one or more embodiments, the high-voltagemulti-filar outer coil 320 can have other types of multi-filarconstructions. The multi-filar construction of the high voltageconductor coil 320 results in a relatively low DC resistance, e.g.,below approximately ten ohms and can be formed from two or moregenerally cylindrically wound filars. Such constructions allow the highvoltage conductor coil 320 to be suitable for use in high voltagedefibrillation lead applications.

In various embodiments, the high-voltage multi-filar outer coil can havea pitch 510 a and a filar thickness 520 a of various dimensions toresult in a desired coil inductance value (e.g., greater than or equalto 0.2 μH/inch) when the high-voltage multi-filar outer coil 320 issubjected to an electromagnetic field at a range of radio frequencies(e.g., 40 MHz to 300 MHz) typical of an MRI scan. In some embodiments,the desired coil inductance value is around 0.5 μH/inch. As discussedabove (e.g., see discussion of FIG. 2B), the impedance and inductance oflead 150 can be advantageously adjusted by the choice of variousstructural features of the lead. Examples of structural featuresinclude, but are not limited to, pitch 510 a, filar thickness 520 a,coil diameter 530 a, and others.

For a typical cylindrically closely wound coil, the inductance of thecoil per unit length can be approximated using the following equation:

$L \approx \frac{\mu_{0}\pi\; a^{2}}{4b^{2}N^{2}}$where μ₀ is the permeability of the free space, a is the mean diameterof the coil 530 a, b is the pitch of the coil 510 a (i.e., distancebetween adjacent filars), and N is the filar count. Based on theequation, coil inductance per unit length is proportionally to thesquare of the radius, and reversely proportional to the square of thepitch and filar count.

FIG. 5B illustrates a portion of the high voltage shocking coil 330 inaccordance with some embodiments of the present invention. In theillustrated embodiment, the high voltage shocking coil 330 is atri-filar coil. However, in one or more embodiments, the high voltageshocking coil 330 can have other types of multi-filar constructions. Themulti-filar construction of the high voltage shocking coil 330 resultsin a relatively low DC resistance and can be formed from two or moregenerally cylindrically wound filars. Such constructions allow the highvoltage shocking coil 330 to be suitable for use in high voltagedefibrillation lead applications.

In some embodiments, the third coil, i.e., shocking coil 330, can have afirst end that is connected to the distal section of the high-voltagemulti-filar outer coil 320 via coupler 350. The third coil 330 can beformed in some embodiments, from two or more generally cylindricallywound filars. According to various embodiments, the filar thickness, thepitch 510 b, and the mean coil diameter 520 b can be configured suchthat the shocking coil 330 has a high impedance value when the shockingcoil 350 is subjected to an electromagnetic field at the range of radiofrequencies (e.g., 40 MHz and 300 MHz) characteristic of an MRI scan. Asdiscussed above (e.g., see discussion of FIG. 2B), the impedance andinductance of lead 150 can be advantageously adjusted by the choice ofvarious structural features of the lead. Examples of structural featuresinclude, but are not limited to, pitch, filar thickness, coil diameter,and others.

FIG. 5C illustrates a portion of the low voltage inner coil 340 inaccordance with some embodiments of the present invention. In theillustrated embodiment, the low voltage inner coil 340 is a uni-filarcoil. However, in one or more embodiments, the low voltage inner coil340 can have other types of multi-filar constructions (e.g., 2-filar,3-filar, etc.). The uni-filar construction of low voltage inner coil 340results in a higher DC resistance, e.g., of approximately two hundredohms. Such constructions allow the low voltage inner coil 340 to besuitable for use in pacing applications.

In some embodiments, the inner conductor coil 340 can be split into twopaths; one for cathode and one for anode for pacing pulses. In oneembodiment, the inner conductor coil 340 has a pitch 510 c, a filarthickness 520 c, and a mean coil diameter 530 c that result in a coildesired impedance value when the inner conductor coil 340 is subjectedto the range of radio frequencies (e.g., 40 MHz and 300 MHz. Asdiscussed above (e.g., see discussion of FIG. 2B), the impedance andinductance of lead 150 and/or lead coils can be advantageously adjustedby the choice of various structural features of the lead. Examples ofstructural features include, but are not limited to, pitch, filarthickness, coil diameter, and others.

In one embodiment, the high voltage conductor coil 320 has a pitch 510 aof about 0.010 inches, four filar count, and a mean coil diameter 530 aof about 0.050 inches, resulting in a coil inductance value of about0.13 μH/inch. The lower limit for high voltage coil is thus set to be0.1 be μH in one embodiment. The inner conductor coil 340 has a pitch510 c of about 0.005 inches, filar count of 1, and a mean coil diameterof about 0.023 inches, resulting in a coil inductance per unit lengthvalue of about 0.5 μH/inch in one embodiment. The lower limit for lowvoltage coil can be set to approximately 0.2 μH in some embodiments. Insome embodiments, the low voltage coil and high voltage coil inductancelimits can be different, while in other embodiments the inductancelimits can be the same.

As discussed above, the designs of various embodiments of the presentinvention can result in significant heat reduction over normal leaddesigns when exposed to MRI related frequencies. In one exemplaryembodiment, a test sample can have a uni-filar low voltage coil madefrom approximately 0.004 inches OD wire. The OD of the coil isapproximately 0.027 inches and the pitch of the coil is close to 0.004inches. The test sample further has a 4 filar high voltage coil, madefrom approximately 0.010 inch wire. The high voltage coil has an OD ofapproximately 0.090 inches and the pitch of the coil is approximately0.012 inches.

FIG. 6 illustrates the resulting temperature increases when a standardlead design and the exemplary lead design is subjected to MRI relatedfrequencies. The total test mule length is 60 cm. The heating tests forthe standard lead design and the exemplary lead design were performedunder the same 64 MHz testing conditions. As can be seen in FIG. 6, theexemplary lead design results in a temperature rise approximately tendegrees less than that of a standard lead at the tip. In addition, theexemplary lead design results in a temperature rise approximately fourdegrees less than that of a standard lead at a ring electrode.

Although the embodiments above describe and illustrate multi-conductorleads with co-axially configured coil conductors, the high inductanceconductor coils 320, 340 can advantageously be employed in other leadconfigurations within the scope of the present invention. For example,FIG. 7 illustrates a transverse cross-sectional view of an alternativeembodiment of the lead 150 utilizing a multi-lumen lead body such as iscommonly employed in conventional defibrillation leads. As shown in FIG.7, the lead body includes an inner tubular member 710 and an outertubular member 720 disposed over and bonded to the inner tubular member710. The tubular members 710, 720 can be made from any number offlexible, biocompatible insulative materials, including withoutlimitation, polymers such as silicone and polyurethane, and copolymersthereof. As further shown, the inner tubular member 710 includes aplurality of lumens 730, 740, 750, and conductors 760, 770, and 780 aredisposed, respectively, in the lumens 730, 740, and 750. Each of theconductors 760, 770, and 780 extends longitudinally within therespective lumen 730, 740, and 750, and is electrically coupled to anelectrode (e.g., the electrodes 360 in FIG. 3) and also to an electricalcontact of the connector assembly 310.

In addition, the inner tubular member 710 may include a greater orlesser number of lumens, depending on the particular configuration ofthe lead 150. For example, the inner tubular member 710 may include agreater number of lumens to house additional conductor wires and/orelectrode coils within the lead 150 for supplying current to othershocking coils and/or pace/sense electrodes.

In the embodiment of FIG. 7, the conductor 760 is configured insubstantially the same manner as the coil conductor 320 described above,and can operate as a low-voltage pace/sense circuit as described above.Accordingly, the conductor 760 advantageously has the samehigh-inductance characteristics described above with respect to theconductor 320. In the illustrated embodiment, the conductors 770, 780are stranded-wire cable conductors which are well known in the art foruse in high voltage applications, e.g., to supply defibrillation stimulito high voltage shocking coils such as the shocking coil 330 in FIG. 3.

The various embodiments of the lead 150 described above, advantageouslyminimize induced currents in the lead conductors resulting from exposureto external MRI electromagnetic fields. This is in contrast toconventional ICD lead systems utilizing stranded cable conductors totransmit the shocking currents from the PG to the shocking electrodes.While such cable conductors provide excellent electrical performance fordelivering anti-tachycardia therapy, stranded cable conductors also havea low impedance and thus are susceptible to generation of inducedcurrents when exposed to an alternating electromagnetic field such asthat present during an MRI scan. The high impedance conductorconfigurations for the lead 150 described above minimize the effects ofMRI radiation while still providing suitable electrical performance foruse in anti-tachycardia therapy applications.

Various modifications and additions can be made to the exemplaryembodiments discussed without departing from the scope of the presentinvention. For example, while the embodiments described above refer toparticular features, the scope of this invention also includesembodiments having different combinations of features and embodimentsthat do not include all of the described features. Accordingly, thescope of the present invention is intended to embrace all suchalternatives, modifications, and variations as fall within the scope ofthe claims, together with all equivalents thereof.

What is claimed is:
 1. An implantable medical device lead comprising: aninner conductor coil comprising one or more generally cylindricallywound filars, wherein the inner conductor coil is configured to have afirst inductance value greater than or equal to 0.2 μH/inch when theinner conductor coil is subjected to a range of radio frequencies; and amulti-filar outer coil comprising two or more generally cylindricallywound filars, wherein the multi-filar outer coil is configured to have asecond inductance value greater than or equal to 0.1 μH/inch when themulti-filar outer coil is subjected to the range of radio frequencies.2. The implantable medical device lead of claim 1, wherein a layer ofinsulation is disposed about at least a portion of the inner conductorcoil.
 3. The implantable medical device lead of claim 1, wherein theinner conductor coil has a unifilar construction.
 4. The implantablemedical device lead of claim 1, wherein the inner conductor coil has anaverage pitch of approximately 0.005 inches.
 5. The implantable medicaldevice lead of claim 4, wherein the inner conductor coil has a unifilarconstruction and a mean coil diameter of 0.023 inches.
 6. Theimplantable medical device lead of claim 1, wherein the inner conductorcoil has an inductance greater than approximately 0.5 μH/inch.
 7. Theimplantable medical device lead of claim 1, wherein the first inductancevalue (L) is set by a number of cylindrically wound filars (N), a pitch(b) of the inner conductor coil, and a mean coil diameter (a) by theequation $L \approx \frac{\mu_{0}\pi\; a^{2}}{4b^{2}N^{2}}$ where μ₀ isthe permeability of the free space.
 8. The implantable medical devicelead of claim 1, wherein the inner conductor coil has a DC resistanceless than 200 ohms.
 9. The implantable medical device lead of claim 1,wherein the multi-filar outer coil is a ribbon-type conductor coil. 10.A medical lead, comprising: a flexible body having a proximal regionwith a proximal end, and a distal region; a connector coupled to theproximal end of the body configured for electrically and mechanicallyconnecting the lead to an implantable pulse generator; an innerconductor coil configured to convey electrical signals between a distalsection and a proximal section of the lead, the low voltage innerconductor coil comprising one or more generally cylindrically woundfilars, wherein the inner conductor coil is configured to have a firstinductance greater than or equal to 0.2 μH/inch when subjected to radiofrequencies between 40 megahertz (MHz) and 300 MHz; and a multi-filarouter conductor coil comprising two or more generally cylindricallywound filars radially surrounding at least a portion of the low voltageinner conductor coil, wherein the multi-filar outer conductor coil isconfigured to have a second inductance greater than or equal to 0.1μH/inch when subjected to the radio frequencies between 40 megahertz(MHz) and 300 MHz.
 11. The medical lead of claim 10, and furthercomprising: a tri-filar shocking coil with a proximal end, wherein theproximal end of the tri-filar shocking coil is connected via a couplerto a distal end of the multi-filar outer coil.
 12. The medical lead ofclaim 11, wherein the multi-filar outer coil has an outer diameterlarger than an outer diameter of the tri-filar shocking coil.
 13. Themedical lead of claim 10, wherein the multi-filar outer conductorcomprises a quad-filar coil having a helix-like shape.
 14. The medicallead of claim 10, wherein the lead further includes one or more layersof insulating material surrounding one or both of the inner conductorcoil and the multi-filar outer conductor coil.
 15. The medical lead ofclaim 10, wherein the inner conductor coil and the multi-filar outerconductor coil have different pitches.
 16. The medical lead of claim 10,wherein the inner conductor coil and the multi-filar outer conductorcoil each have a pitch no greater than about 0.005 inch (0.127 mm). 17.An implantable medical device lead comprising: an inner conductor coilcomprising one or more wound filars and configured to have a firstinductance value greater than or equal to 0.2 μH/inch when subjected toradio frequencies between 40 megahertz (MHz) and 300 MHz; a multi-filarouter conductor coil comprising two or more generally cylindricallywound filars radially surrounding at least a portion of the innerconductor coil, wherein the multi-filar outer conductor is configured tohave a second inductance value greater than 0.1 μH/inch when subjectedto the radio frequencies between 40 megahertz (MHz) and 300 MHz; and atri-filar shocking coil with a proximal end, wherein the proximal end isconnected via a coupler to a distal end of the multi-filar outerconductor coil.
 18. The implantable medical device of claim 17, whereinthe inner conductor coil, the multi-filar outer conductor coil, and thetri-filar shocking coil have different pitches.
 19. The implantablemedical device of claim 17, wherein the lead further includes one ormore layers of insulating material surrounding one or more of the lowvoltage inner conductor coil, the multi-filar high voltage outerconductor coil, and the tri-filar shocking coil.
 20. The implantablemedical device of claim 17, wherein a pitch of the multi-filar outerconductor coil is about 0.010 inches, a mean coil diameter of themulti-filar outer conductor coil is about 0.090 inches and results in acoil inductance value of about 0.13 μH/inch, and a pitch of the innerconductor coil is about 0.005 inches, the inner conductor coil is formedfrom one cylindrically wound filar, and a mean coil diameter of theinner conductor coil is about 0.023 inches resulting in a coilinductance per unit length value of about 0.5 μH/inch.